1. Field of the Invention
The present invention relates to a method for producing a semiconductor device.
2. Description of Related Art
When a semiconductor device that uses silicon carbide as, a semiconductor material is produced, a silicon carbide wafer may be heat treated. For example, after a conductive impurity was introduced in the silicon carbide wafer, an annealing treatment is conducted to activate the introduced conductive impurity. When the silicon carbide wafer is heat treated, there may be a problem that Si in the silicon carbide wafer sublimes to roughen a surface, or a concentration of conductive impurity on a wafer surface (hereinafter, simply referred to also as “impurity concentration”) is degraded. In particular, in the annealing treatment to activate the conductive impurity, the silicon carbide wafer is heat treated at high temperatures such as 1500° C. to 1900° C., therefore, the above-described problem becomes remarkable. In this connection, Japanese Patent Application Publication No. 2009-266969 (JP 2009-266969 A) discloses a technology where the annealing treatment is performed after a cap layer has been formed on a surface of a silicon carbide substrate. The cap layer has a melting point higher than a temperature of the annealing treatment and is formed of metal carbide that is oxidized at 1000° C. or less. Further, the annealing treatment is performed under deoxidation atmosphere.
According to the technology of JP 2009-266969 A, the surface of the silicon carbide substrate is protected by the cap layer. In addition, since the annealing treatment is performed under the deoxidation atmosphere, even when an oxidation start temperature of the metal carbide that forms the cap layer is lower than a temperature of the annealing treatment, the cap layer can be hindered from being oxidized. Therefore, it is said that the problem that the surface of the silicon carbide substrate is roughened by the annealing treatment or an impurity concentration is degraded can be hindered from occurring. In addition, it is said that since the cap layer is oxidized at 1000° C. or less to be brittle, when an oxidizing treatment is performed at a predetermined temperature after the annealing treatment, the cap layer can be readily removed.
In the technology of JP 2009-266969 A, the steps of forming and removing the cap layer are added to the producing step of the semiconductor device. As the result, the production efficiency of semiconductor devices may be degraded or the production cost may increase. Further, it is said that the cap layer becomes brittle by oxidation and can be readily removed. However, depending on removing methods, it cannot be denied that residues of the cap layer may remain. When the residue of the cap layer remains on a surface of the silicon carbide substrate, properties of the semiconductor device may be adversely affected.